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Abstract:

Multiple-Input Multiple-Output (MIMO) systems and methods are provided
for enabling network MIMO among base stations (BSs) and a mobile station
(MS). A BS configure a network MIMO zone based on an indication of at
least one other BS eligible for network MIMO. The network MIMO zone is
defined by resources having at least time and frequency dimensions
allocated for master transmission under control of the BS, or slave
transmission under control of the other BS eligible for network MIMO. The
BS can transmit data on the network MIMO zone to the MS. Where there is
no data to transmit to the MS, the BS may transmit data on the network
MIMO zone to another MS. To configure the network MIMO zone, the BS may
look up in a table combinations of the eligible BSs associated with the
indication.

Claims:

1. A mobile station (MS) in a Multiple-Input Multiple-Output (MIMO)
system for enabling network MIMO among the MS and a plurality of base
stations (BSs), the MS comprising: at least one transmitting antenna; and
a control system configured to: determine BSs eligible for network MIMO;
transmit an indication of the BSs eligible for network MIMO to at least
one of the BSs eligible for network MIMO; and receive data on a network
MIMO zone from at least one antenna of a BS of the BSs eligible for
network MIMO, the network MIMO zone being defined by a resource allocated
for master transmission under control of the BS, or slave transmission
under control of another BS eligible for network MIMO.

2. The MS of claim 1, wherein the control system is further adapted to:
for open loop transmissions: receive pilot symbols from the BS and the
another BS eligible for network MIMO; measure Carrier to Interference
Ratios (C/Is) based on the pilot symbols; and transmit the C/Is to at
least one of the BSs eligible for network MIMO.

3. The MS of claim 1, wherein the control system is further adapted to:
for closed loop transmissions: receive pilot symbols from the BS and the
another BS eligible for network MIMO; measure C/Is based on the pilot
symbols; and transmit to at least one of: the BSs eligible for network
MIMO the C/Is, a precoding matrix index, or a rank indicating streams for
receiving data.

4. The MS of claim 1, wherein the control system is further adapted to:
determine whether the MS is ready to receive network MIMO transmission
based on at least one of: an absolute C/I threshold; a difference in the
C/Is between the BS and the another BS eligible for network MIMO;
instantaneous channel conditions; average channel conditions; or the
structure of the MS.

[0003] In a MIMO communication system, a transmitter transmits data
through multiple transmitting antennas and a receiver receives data
through multiple receiving antennas. The data to be transmitted is
usually divided between the transmitting antennas. Each receiving antenna
receives data from all the transmitting antennas, so if there are M
transmitting antennas and N receiving antennas, then the signal will
propagate over M×N channels, each of which has its own channel
response. The movement of the receiver in relation to the transmitter
results in significant fluctuation in channel conditions. The multiple
antennas provide spatial diversity for communications. Typically, if the
receiver requires a large transmission power for data, for example a
receiver that is geographically located at the edge of a communication
cell, the receiver is attended to by using a different transmission than
receivers in closer proximity to the transmitter.

[0004] In order to improve coverage and throughput, network MIMO can be
used. In network MIMO, each receiver is in network MIMO communication
with multiple transmitters.

SUMMARY

[0005] In accordance with a broad aspect, there is provided in a
Multiple-Input Multiple-Output (MIMO) system, a method of enabling
network MIMO among a plurality of base stations (BSs) and at a least one
mobile station (MS), the method comprising: configuring at a BS a network
MIMO zone based on an indication of at least one other BS eligible for
network MIMO, the network MIMO zone being defined by resources having at
least time and frequency dimensions allocated for master transmission
under control of the BS, or slave transmission under control of one of
the at least one other BS eligible for network MIMO.

[0006] In some embodiments, the method further comprises transmitting data
on the network MIMO zone on at least one antenna of the BS to the at
least one MS.

[0007] In some embodiments, the method further comprises, where there is
no data to transmit to the at least one MS, transmitting data on the
network MIMO zone on at least one antenna of the BS to a MS other than
the at least one MS.

[0008] In some embodiments, the network MIMO zone is further defined by
resources allocated using a channelization procedure that is the same as
a channelization procedure of the one of the at least one other BS
eligible for network MIMO.

[0009] In some embodiments, where there is no data to transmit to the at
least one MS, the network MIMO zone is further defined by resources
allocated using a BS specific channelization procedure.

[0010] In some embodiments, the network MIMO zone is further defined by
contiguous tones that are in alignment with contiguous tones of a network
MIMO zone of the one of the at least one other BS eligible for network
MIMO.

[0011] In some embodiments, the network MIMO zone is defined by resources
having a further dimension of code spreading allocated for master
transmission under control of the BS, or slave transmission under control
of one of the at least one other BS eligible for network MIMO.

[0012] In some embodiments, configuring at the BS a network MIMO zone
based on the indication comprises looking up in a table combinations of
the at least one BS eligible for network MIMO associated with the
indication.

[0013] In some embodiments, the method further comprises determining
scheduling information indicating at least one of: an identifier of one
of the at least one MS, a location of the network MIMO zone, an
identifier of the BS, a resource assignment, and a hopping pattern; and
transmitting the scheduling information.

[0014] In some embodiments, the method further comprises transmitting a
signal to the at least one MS specifying that the at least one MS is
eligible for network MIMO.

[0015] In some embodiments, at least one of the at least one MS is a
cell-edge MS.

[0016] In some embodiments, the method further comprises transmitting on
the network MIMO zone common pilot symbols that are orthogonal for the BS
and the one of the at least one other BS eligible for network MIMO.

[0017] In some embodiments, the method further comprises performing a
Hybrid Automatic Repeat Request (HARQ) re-transmission protocol, wherein
the HARQ re-transmission protocol is selected from at least one of:
asynchronous, synchronous, and resource adaptive synchronous (RAS)-HARQ.

[0018] In some embodiments, performing the HARQ re-transmission protocol
comprises cycling through the BS and the one of the at least one other BS
eligible for network MIMO between re-transmissions such that data is
re-transmitted to the at least one MS in subsequent re-transmissions.

[0019] In accordance with another broad aspect, there is provided a base
station (BS) in a Multiple-Input Multiple-Output (MIMO) system for
enabling network MIMO among a plurality of BSs and at a least one mobile
station (MS), the BS comprising: at least one transmitting antenna; and a
control system adapted to: configure a network MIMO zone based on an
indication of at least one other BS eligible for network MIMO, the
network MIMO zone being defined by resources having at least time and
frequency dimensions allocated for master transmission under control of
the BS, or slave transmission under control of one of the at least one
other BS eligible for network MIMO.

[0020] In some embodiments, the control system is further adapted to:
allocate data on the network MIMO zone on at least one antenna of the BS
for transmission to the at least one MS.

[0021] In accordance with still another broad aspect, there is provided in
a mobile station (MS), a method of enabling network Multiple-Input
Multiple-Output (MIMO) among a plurality of base stations (BSs) and the
MS, the method comprising: determining at the MS BSs eligible for network
MIMO; transmitting an indication of the BSs eligible for network MIMO to
at least one of the BSs eligible for network MIMO; and receiving data on
a network MIMO zone from at least one antenna of a BS of the BSs eligible
for network MIMO, the network MIMO zone being defined by a resource
allocated for master transmission under control of the BS, or slave
transmission under control of another BS eligible for network MIMO.

[0022] In some embodiments, the method further comprises for open loop
transmissions: receiving pilot symbols from the BS and the another BS
eligible for network MIMO; measuring Carrier to Interference Ratios
(C/Is) based on the pilot symbols; and transmitting the C/Is to at least
one of the BSs eligible for network MIMO.

[0023] In some embodiments, the method further comprises for closed loop
transmissions: receiving pilot symbols from the BS and the another BS
eligible for network MIMO; measuring C/Is based on the pilot symbols; and
transmitting to at least one of the BSs eligible for network MIMO the
C/Is, a precoding matrix index, and a rank indicating streams for
receiving data.

[0024] In some embodiments, the method further comprises determining
whether the MS is ready to receive network MIMO transmission based on at
least one of: an absolute C/I threshold; a difference in the C/Is between
the BS and the another BS eligible for network MIMO; instantaneous
channel conditions; average channel conditions; and the structure of the
MS.

[0025] Other aspects and features of the present invention will become
apparent, to those ordinarily skilled in the art, upon review of the
following description of the specific embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The invention will now be described in greater detail with
reference to the accompanying diagrams, in which:

[0027] FIG. 1 depicts a MIMO communication system for network MIMO
transmissions in accordance with an embodiment;

[0028] FIG. 2 depicts transmission of a look-up table of possible
combinations of network MIMO groups for a mobile station;

[0035] FIG. 10 is a flowchart of a method, in a system, of enabling
network MIMO among a plurality of base stations and at a least one mobile
station; and

[0036] FIG. 11 is a flowchart of a method, in a mobile station, of
enabling network MIMO among a plurality of base stations and at a least
one mobile station.

DETAILED DESCRIPTION

[0037] In a MIMO system, a base station (BS) provides communication
services for a coverage area or cell in a wireless communication system.
The term "base station" can refer to any access point providing coverage
to an area. The BS transmits communication signals to mobile stations
(MSs) via multiple antennas. MSs are also commonly referred to as user
terminals, user equipment, and communication devices, for instance. The
term "mobile station" can refer to any receiving device (stationary or
mobile). At a MS side, multiple receive antennas are employed for each
MS.

[0038] FIG. 1 shows a MIMO communication system 10 for network MIMO. MIMO
communication system 10 includes BSs BSA 100, BSB 102, BSC
104 and MSs MS1 106, and MS2 108. Each of MS1 106 and
MS2 108 can be a wireless device, such as a cellular telephone, a
computer with a wireless modem, or a PDA (personal Digital Assistant).

[0039] Each of BSA 100, BSB 102, BSC 104 serves a
particular cell to facilitate communication with one or more MSs located
within the cell associated with the corresponding BS. MS1 106 is
linked to BSA 100, and MS2 108 is linked to BSC 104. Thus,
BSA 100 and BSC 104 are the serving BSs for MS1 106 and
MS2 108, respectively. MS1 106 is located at the intersection
of two cells, as is MS2 108. As such, they are known as "cell-edge"
MSs. It is to be understood that cell-edge MSs are a particular example,
and embodiments are not limited to cell-edge MSs.

[0040] In operation, network MIMO combines antennas from BSs located in
neighboring cells to transmit multiple streams to one or more MSs. Thus,
each MS is in network MIMO communication with multiple BSs, including its
serving BS. More particularly, in network MIMO MS1 106 can be in
network MIMO communication with BSA 100 and either BSB 102 or
BSC 104. MS2 108 can be in network MIMO communication with
BSC 104 and either BSA 100 or BSB 102.

[0041] In order to support network MIMO, various features are provided by
the embodiments described herein: network MIMO co-ordination, feedback,
network MIMO with backhaul communications, mobile-assisted network MIMO,
network MIMO zone, HARQ re-transmission, and precoded network MIMO. These
features are described below.

[0042] The following describes network MIMO co-ordination. As noted above,
in network MIMO each MS is in network MIMO communication with multiple
BSs. To determine which BSs are in network MIMO communication with a
particular MS, each MS is provided with an "active set". The active set
of a MS indicates eligible BSs for network MIMO transmission with that
particular MS. The active set may be stored or updated by a MS from time
to time. In the example of FIG. 1, MS1 106 and MS2 108 each
have an active set that indicates that BSA 100, BSB 102, and
BSC 104 are eligible for network MIMO transmission.

[0043] An active set can be based on any number of considerations, such as
the best BSs to be used from a signal strength, proximity, or
interference standpoint. For example, the active set of a MS can be based
on the signal strength of the preamble of a signal received by the MS. It
is to be understood that generation of the active set is not limited to
these considerations.

[0044] Generally, multiple BSs participating in network MIMO with a MS can
be collectively referred to as a "network MIMO group" for a particular
MS. The network MIMO group can be a subset of the MS's active set. BSs
participating in network MIMO transmission can be referred to as
"participating BSs". In FIG. 1, MS1 106 has a network MIMO group 112
consisting of BSA 100 and BSB 102. MS2 108 has a network
MIMO group 114 consisting of BSB 102 and BSC 104. Although in
FIG. 1 the network MIMO group of MS1 106 and MS2 108 is a
subset of the MS's active set, this does not have to be the case.

[0045] The active set can indicate BSs eligible for network MIMO download
and upload. If such download and upload active sets are used, the network
MIMO group contains all or a subset of the BSs that are the union of the
download and upload active sets. The upload and download groups can be
different or the same.

[0046] As shown in FIG. 1, BSA 100 has a control system 115 which is
adapted to configure a network MIMO zone. Network MIMO zones are
described in more detail below with reference to FIG. 5. Each BS can have
a control system.

[0047] An MS determines a network MIMO group based on its active set. An
MS may determine the network MIMO group using a look-up table of possible
combinations of network MIMO groups, based on the MS's active set. Many
combinations are possible. FIG. 2 depicts transmission of a look-up table
116 of possible combinations of network MIMO groups for MS1 106. The
second column of the look-up table 116 defines a combination of network
MIMO groups, and the first column defines indices such that each index is
associated with a respective combination. Various indexing schemes are
possible. According to one indexing scheme, if there is a maximum of N
BSs in the network MIMO group, then the look-up table contain
combinations of the N-1 BSs, excluding the serving BS. This keeps the
number of possible combinations smaller, such that the combinations can
be signaled with less overhead. However, the combinations in the table
could be extended to include the serving BS. If the serving BS is
excluded from the combinations, the number of rows in the look-up table
would be 2.sup.(N-1)-1. Such an indexing scheme has been used for the
look-up table 116. Referring to FIG. 2, MS1 106 can have a maximum
of 3 BSs in any of its network MIMO groups, so the look-up table 116
contains combinations of 3-1=2 BSs, excluding serving BSA 100.
Look-up table 116 has 2.sup.(3-1)-1=3 rows. Referring to the first row, a
network MIMO group consisting of BSB 102 is associated with index
"0". Referring to the second row, a network MIMO group consisting of
BSC 104 is associated with index "1". Referring to the third row, a
network MIMO group consisting of BSB 102 and BSC 104 is
associated with index "2".

[0048] MS1 106 generates the look-up table 116 based on its active
set. The look-up table 116 is stored at MS1 106 and its serving
BSA 100. Alternatively, the look-up table 116 is generated
independently at serving BSA 100 by following certain rules. For
example, if MS1 106 sends a network MIMO group in a certain order,
the look-up table can be generated according to such order. Although in
this example the rule for generating the table depends on the order in
which MS1 106 reports the network MIMO group, other rules may be
used.

[0049] After generating the look-up table 116, MS1 106 determines a
network MIMO group and can transmit an index associated with the network
MIMO group. The index can be transmitted on an uplink (UL) feedback
channel. Transmitting the index can require N-1 bits, which may not be
large in practical network MIMO communication systems.

[0050] In the example of FIG. 2, if MS1 106 decides to receive
network MIMO transmissions from BSA 100 and BSB 102, then
MS1 106 looks up the combination BSB 102 in the look-up table
116, notes that this combination is associated with index "1", and
transmits index "1". MS1 106 transmits the index to its serving
BSA 100, which relays the index to the remaining BSs in the network
MIMO group, namely BSB 102 and BSC 104 (illustrated using solid
lines). Alternatively, MS1 106 broadcasts the index to the network
MIMO group, namely BSA 100 and BSB 102 (illustrated using
broken lines). The index can be transmitted on a UL feedback channel. In
addition to an index, MS identification data, such as a MS identifier
(ID), can also be transmitted to a BS in order to allow that BS to decode
information from and relay information to the MS. Whenever the network
MIMO group of a MS changes, for example due to a change in the active set
of that MS, its serving BS sends the updated look-up table to the
remaining BSs in the updated active set.

[0051] Upon receipt of the index from MS1 106, a BS retrieves the
corresponding network MIMO group, and prepares for network MIMO
accordingly. For example, BSB 102 will note that it is listed in the
network MIMO group and prepares for network MIMO with MS1 106, while
BSC 104 notes that it is not listed in the network MIMO group and
therefore does not prepare for network MIMO with MS1 106.

[0052] The above described network MIMO co-ordination, including
determining a network MIMO group based on an active set. Before such
determination, a MS can be involved in feedback to assist in making that
determination. The following describes such feedback.

[0053] MSs that are eligible for network MIMO are sent an indication of
eligibility for network MIMO. Such indications can be sent to MSs within
a cell, cell-edge MSs, or both.

[0054] Upon receiving an indication of network MIMO, a MS measures C/I
(Carrier to Interference Ratio) based on pilots the MS has received.

[0055] The MS determines whether it is ready to receive network MIMO
transmission. The determination can be based on various factors. Three
example factors are described below.

[0056] The determination can be based on an absolute C/I threshold and/or
a difference in the C/Is of a MS's neighboring BSs, since the MS has full
knowledge of the channel conditions of the BSs in the active set.

[0057] The determination can be based on instantaneous or average channel
conditions. In this case, a subset of the BSs could be in the active set.

[0058] The determination can be based on the MS's receiver structure, for
example Minimum Mean-Squared Error (MMSE) or Successive Interference
Cancellation (SIC) with MMSE. With SIC, the MS can decide to do network
MIMO when there is a larger differential in the C/I between serving BS
and the remaining BSs in its active set than in the case where SIC is not
used.

[0059] If the MS determines that it is not ready, the MS does not send a
network MIMO indication. If the MS determines that it is ready, the MS
sends feedback information, including a network MIMO indication,
information on the C/Is, an index, and an indication of MIMO mode. MIMO
mode refers to the type of MIMO transmission, such as open-loop,
closed-loop, blast, SM (spatial multiplexing), and STTD (space-time
transmit diversity). The MS can also send a preferred matrix index to
indicate its choice of precoding.

[0060] The MS sends the feedback to all BSs in the network MIMO group, or
a subset of it.

[0061] The serving BS designates UL resources for the feedback
information. The serving BS signals the location of the UL resources to
the network MIMO group so that the network MIMO group can retrieve the
feedback information. Since only a subset of the BSs in the network MIMO
group may be involved in an actual network MIMO transmission, the MS
sends the index on the uplink.

[0062] Since the uplink and downlink active sets may be different, the UL
feedback can be sent in two ways. If the participating BSs are in the
uplink active set, the UL feedback channel can be adjusted so that it
targets the participating BSs to hear the feedback. For example, if the
participating BSs are in the uplink active set (e.g. in TDD (Time
Division Duplex)), the UL feedback channel can be adjusted in terms of
power. If the participating BSs are not in the uplink active set, the
serving BS is responsible for decoding the UL feedback.

[0063] The BS can encode transmissions vertically or horizontally, as
described below.

[0064] For vertical encoding or STTD, one C/I is reported. The C/I channel
is scrambled by the MS ID. Since one encoded packet can be sent to
multiple streams in vertical encoding, the participating BSs can encode
the same data independently (since they have already been receiving
copies of the MS data) and extract the portion of the encoded data for
transmission. Alternatively, the serving BS encodes the data and sends
the portion of the encoded data to the BSs in the network MIMO.

[0065] For horizontal encoding, C/Is for different streams are reported
for each participating BS. A C/I channel can be scrambled by the MS ID.
The BSs that will participate in the network MIMO transmission to the MS
may be signaled by the index of the look-up table. The order of the C/I
report for each stream (whether it is encoded separately or jointly)
corresponds to the order in the entry of the look-up table.

[0066] The serving BS performs Modulation and Coding Scheme (MCS)
selections based on the reported C/Is and signals the other participating
BSs the MCS. Alternatively, MCS selection can be performed independently
by the participating BSs.

[0067] FIG. 3 depicts a representative example of one possible frame
diagram of network MIMO with backhaul communication. MS1 106,
BSA 100 and BSB 102 can communicate with each other over frames
N to N+8. As noted above, MS1 106 measures C/Is for the network
MIMO. MS1 106 feedbacks the C/Is to the serving BSA 100.
Serving BSA 100 can send to the other participating Bs, namely
BSB 102, scheduling information, which includes information on
resource allocation, the MCS, the MIMO mode, and the transmission time
(e.g. frame number with the appropriate offset for the different BSs).
Alternatively, the scheduling can be performed by each participating BS
individually.

[0068] Serving BSA 100 can also send an indication of eligibility for
network MIMO.

[0069] BSB 102 can send an indication of participation in the network
MIMO, for instance an acknowledgement (e.g. an ACK) to indicate
participation, or a negative acknowledgement (e.g. a NACK) to indicate no
participation for example due to loading in the BSs.

[0070] If serving BSA 100 does not receive a response from MB
102, then BSA 100 can assume that the indication of eligibility was
not received by BSB 102. In that case, BSA 100 can resend the
indication of eligibility.

[0071] Where there are only two BSs in the network MIMO group, such as
BSA 100 and BSB 102, serving BSA 100 can change the MIMO
mode if BSB 102 does not respond or a NACK is received. For example,
an STTD rate 2 can be changed to STTD rate 1.

[0074] Mobile-assisted network MIMO eliminates backhaul signaling among
BSs. It deals with network MIMO when at least one participating BS is
from another cell.

[0075] The MS is used to relay information. Together with short frame
duration, mobile-assisted Network MIMO reduces the scheduling delay and
enables more dynamic scheduling.

[0076] In every superframe, a set of resources for network MIMO
transmission, known as a network MIMO zone, is configured. Network MIMO
zones are described in more detail below with reference to FIG. 5.
Participating BSs are determined based on MS feedback.

[0077] Referring to FIG. 4, the steps of MS1 106 measuring C/Is for
the network MIMO and feeding back the C/Is to the master BSA 100 are
the same as in FIG. 3.

[0078] Afterwards, the serving BSA 100 schedules MS1 106 in the
network MIMO zone using a scheduler. A fixed amount of time is allowed to
elapse between this decision of the scheduler and actual transmission,
for instance 5 frames from N+4 to N+8 in the example of FIG. 4.

[0080] BSB 102 sends an indication of participation in the network
MIMO, such as an ACK, to MS1 106, so that MS1 106 can be ready
to decode data from BSB 102. This allows MS1 106 to determine
how many participating BSs successfully receive the indication of
eligibility for network MIMO. If MS1 106 does not receive an
ACK/NACK from BSB 102, it assumes that the indication of eligibility
for network MIMO was not received by BSB 102. The MS1 106 sends
the scheduling info through higher-layer signaling to re-synchronize the
data.

[0081] Finally, BSA 100 and BSB 102 transmit data to MS1
106.

[0082] FIG. 5 is an example schematic diagram of three network MIMO zones
for each BS. Generally, resources for network MIMO transmission are
referred to as a "network MIMO zone". The network MIMO zone can be
described as a set of two dimensional resources (time and frequency),
though in some embodiments code spreading can be used to provide a third
dimension. The network MIMO zone can be a TDM (Time Division
Multiplexing)-based zone, an FDM (Frequency Division Multiplexing)-based
zone, or a combined TDM/FDM-based zone.

[0083] Referring to FIG. 5, shown are resources 117,118,119 for BSA
100, BSB 102, and BSC 104, respectively. These resources are
shown as having a two dimensional appearance in which the horizontal
direction is frequency and the vertical direction is time. The resources
for each BS are partitioned into 3 zones, which can be used for network
MIMO transmission. In the example of FIG. 5, a MS is in network MIMO
communication with BSA 100, BSB 102, and BSC 104 of FIG.
1.

[0084] Shown are various types of partitions. For each resource, a BS
known as the "master" BS transmits, and other participating BSs transmit
as "slaves". For illustration purposes, a network MIMO zone assigned to a
master for network MIMO transmission is illustrated with dense stripes or
dotting. A network MIMO zone assigned to a slave for network MIMO
transmission is illustrated with light stripes or dotting. A network MIMO
zone assigned to a BS for non-network-MIMO transmission is illustrated
with blank space. If there is no data transmission in a network MIMO
zone, non-network MIMO transmissions can be scheduled to avoid wastage of
resources.

[0087] The same channelization procedure and hopping pattern arrangement
can be used. Thus, the allocation of sub-resources to users is the same
for all of the network MIMO zones for a network MIMO group. However, when
no network MIMO is scheduled, BS specific channelization procedure and
hopping pattern can be used.

[0088] The size of the network MIMO zone can be configured every
superframe based on the number of network MIMO users.

[0089] FIGS. 6 and 7 depict exemplary scattering of pilot signals. BSs
send pilots signals, which the MSs receive and use for channel
estimation. Common pilots that are orthogonal can be used. This can
facilitate channel estimation and precoder selection for closed loop
network MIMO.

[0090] The pilots can be sent for all antennas, or for a subset of
antennas with cycling. Such pilot subset cycling can reduce pilot
overhead. Alternatively, other pilots could be used, for example
dedicated pilots.

[0091] FIG. 6 depicts an embodiment where 2 BSs send pilots for all
antennas. Shown on the left side is a network MIMO zone in which pilots
for all of the two antennas of BSA 100 of FIG. 1 have been
scheduled, namely pilots 140,142,144,146. Shown on the right side is a
network MIMO zone in which pilots for all of the two antennas of BSB
102 of FIG. 1 have been scheduled, namely pilots 148,150,152, 154.

[0092] In operation, participating BSs send pilots to a MS. The MS
receives the pilots, measures the C/Is for the pilots, estimate channels
for all antennas, and report on the C/Is.

[0093] The MS can be configured to receive data transmission from the
antennas of the BSs and report on their C/Is in any number of ways. More
specifically, the MS can be configured to receive data transmission from
all antennas, data transmission with antenna hopping, or data
transmission with antenna selection.

[0094] In the case of data transmission from all antennas, the MS reports
one or multiple C/Is for all antennas, and BSs transmit data from all
antennas. Transmission at an STTD (Space-time block coding based transmit
diversity) rate 2 can be used.

[0095] In the case of data transmission with antenna hopping, the MS
reports one or multiple C/Is for all antennas, and BSs transmit data on a
subset of the antennas with a pre-defined hopping pattern which hops
around all antennas. Transmission at an STTD rate 1 with antenna hopping
can be used.

[0096] In the case of data transmission with antenna selection, the MS
reports one or multiple C/Is for a subset of antennas. The BSs transmit
data on the subset of antennas.

[0097] Since pilots are sent for all transmit antennas and the MS needs to
estimate channels for all antennas, the embodiment of FIG. 6 can
represent a higher overhead and computation complexity. However, it can
provide full flexibility to achieve spatial diversity.

[0098] FIG. 7 depicts an embodiment where two BSs send a subset of pilots
and cycle the pilots in a regular interval. Only a subset of pilots is
sent on a network MIMO zone to reduce the pilot overhead. More
specifically, BSA 100 of FIG. 1 sends a subset consisting of pilots
140,142, and later a subset consisting of pilots 144,146. BSB 102 of
FIG. 1 sends a subset consisting of pilots 140,142, and later a subset
consisting of pilots 144,146.

[0099] Pilots are cycled in a pre-defined pattern in a regular interval
for additional spatial diversity. In order to enable proper C/I reporting
and channel estimation for data demodulation, the pilot cycling pattern
can be configured to be changed only every superframe so that within the
superframe, a MS reports and estimates channels from the same set of
pilots.

[0100] The MS measures the C/Is from all pilots.

[0101] As with the embodiment of FIG. 6, in FIG. 7 the MS can be
configured for receiving data transmission from all antennas, data
transmission with antenna hopping, or data transmission with antenna
selection.

[0102] FIG. 8 depicts a diagram of a HARQ re-transmission scheme.
Re-transmission can be either synchronous or asynchronous.

[0103] A network MIMO resource assignment can be persistent until either
the packet is correctly received or N packets are correctly received. A
MS may have enough data for consecutive transmissions without additional
signaling.

[0104] The data can be cycled through the BSs in re-transmissions for
additional diversity such that the MS receives all the data in subsequent
re-transmissions even if only the serving BS is transmitting, for example
in STTD rate 2,4. Instead of STTD, Space-Time Coding (STC) could be used.

[0105] In the example of FIG. 8, in an initial transmission, a first set
of data (shown as symbols s1, s2-s1*, and s1*) is
sent by a master BS and a second set of data (shown as symbols s3,
s4, -s4*, and s3*) is sent by a slave BS. In the first
re-transmission, the second set of data is sent by the master BS and the
first set of data is sent by the slave BS.

[0106] As noted above in respect of FIG. 2, the BS can encode
transmissions vertically or horizontally.

[0107] In vertical encoding, one ACK/NACK is used for each transmission.
The MS decodes the data successfully and sends an ACK to the BSs. All BSs
receive the ACK. BSs can schedule other users in the resource. Where at
least one BS does not receive the ACK or mistake it as a NACK, these BSs
retransmit the data and do not hear the ACK/NACK again. In other words,
they abort re-transmission.

[0108] Where the MS is unable to decode the data, the MS sends a NACK to
the BSs. All BSs receive the NACK. The BSs retransmit on the same
partition and HARQ interlace and cycle the data if necessary. Where at
least one BS does not receive the NACK, these BSs retransmit and cycle
the data if necessary.

[0109] Where at least one BS mistakes the NACK as an ACK, the BS schedules
non-network MIMO users in a network MIMO zone. The MS may still
soft-combine all data, but half of the data will corrupted. To resolve
this issue, a re-transmission indicator (e.g. 1 bit) can be sent from
each BS to indicate the presence of re-transmission from that particular
BS. For example, the MS only soft-combines data from BSs with the
indicator set to 1. Alternatively, the MS blindly detects the received
signal (after soft-combining) by assuming that the received signal either
contains the re-transmission or not.

[0110] In horizontal encoding, one ACK/NACK is used for each layer. The
sections of data are cycled for redundancy. When one layer finishes
re-transmission, it can retransmit the data in another layer so that it
can be soft combined for diversity (e.g. SFN transmission). For example,
in 2-layer spatial multiplexing, when the first layer is received
successfully while the other layer is not, the same data for the 2nd
layer can be transmitted on the first layer in subsequent
re-transmissions.

[0111] The following describes precoding in network MIMO. In a network
MIMO zone, orthogonal common pilots facilitate channel estimation of the
network MIMO channel and joint precoder selection. The joint precoder can
be selected in many ways, for example by the serving BS, the other
participating BSs, or the MS. The serving BS can select the joint
precoder based on sounding. The MS can select the joint precoder based on
feedback.

[0114] If the precoder is a matrix of size
Ntx--.sub.total×Nstreams, and each BS has Ntx
transmit antennas, then the precoder is divided into blocks of an
Ntx×Nstreams submatrix with the master BS using the first
block, and the second BS using the second block. The order is already
determined by the look-up table 180, and the index is signaled by the MS
as described in respect of FIG. 2.

[0115] To further illustrate network MIMO, a very specific example of a
network MIMO communication system is set forth below.

[0116] In a network MIMO zone, the pilot pattern used can be a four
antenna pattern in accordance with C802.16m-08/172r1. Each BS transmits
pilots for 2 different antennas.

[0117] The HARQ re-transmission is asynchronous, synchronous, or RAS-HARQ.

[0118] A permutation index can be used to signal the resource partition
within the network MIMO zone in accordance with C802.16m-08/176r1.

[0119] A diversity zone or a localized zone is used, as described below.

[0120] In a diversity zone, a network MIMO zone is defined by using the
same channelization procedure as for Fractional Frequency Reuse (FFR). A
FFR zone corresponding to reuse one is used for network MIMO. A common
hopping pattern is used by the coordinating BSs in this zone. If there is
no MS eligible for network MIMO transmission, BS specific hopping pattern
is used and non-network MIMO MSs are scheduled.

[0121] In a localized zone, localized zones between coordinating BSs are
physically aligned. Network MIMO is transparent to the user in the case
of asynchronous HARQ. In synchronous HARQ or RAS-HARQ, only the timing of
the re-transmissions is different in network MIMO to account for the
delay associated with coordinating the transmissions. The C/I measurement
pilots are located on the same tones as in the case of a network MIMO
zone for non-network MIMO transmission. The control information is the
same as in the network MIMO zone for non-network MIMO transmission.

[0122] In terms of procedure, a BS configures a network MIMO zone with a
neighboring BS. The location of the network MIMO zone, the coordinating
BS ID, and the hopping pattern are signaled in a superframe header. The
BS schedules a user in the network MIMO zone. The BS coordinates various
aspects with the participating BSs supporting the serving BS, including
user selection and resource assignment. The BS sends control information
and transmission data to the MS. Re-transmission can occur either inside
or outside the network MIMO zone.

[0123] A MS reports its active set to its serving BS, which can be based
on signal strength, or a static determination. This indicates which BSs
can be used for network MIMO transmission. For an open loop, the MS
measures and reports the C/I for STTD or SM (spatial multiplexing) to the
serving BS. For a closed loop, the MS measures and reports a precoding
matrix index (PMI), a rank and C/I to the serving BS. The rank generally
refers to the number of streams that the MS is able to receive. The MS
decodes the control and transmitted data, and sends an ACK/NACK to the
serving BS.

[0124] FIG. 9 is a plot relating to network MIMO precoding. The plot shows
cumulative distribution functions (CDF) against Signal to Noise Ratios
(SNR) for various scenarios. FIG. 9 shows the possible gain of network
MIMO over 2×2 CL (closed loop) MIMO and 4×2 CL MIMO. It also
shows the gain over the case where the most dominant interferer is not
transmitting.

[0125] FIG. 10 is a flowchart of a method 180, in a system, of enabling
network MIMO among a plurality of BSs and at a least one MS. Step 182
involves configuring at a BS a network MIMO zone based on an indication
of at least one other BS eligible for network MIMO. The network MIMO zone
is defined by resources having at least time and frequency dimensions
allocated for master transmission under control of the BS, or slave
transmission under control of one of the at least one other BS eligible
for network MIMO.

[0126] FIG. 11 is a flowchart of a method 200, in a MS, of enabling
network MIMO among a plurality of BSs and at a least one MS. Step 202
involves determining at the MS BSs eligible for network MIMO. Step 204
involves transmitting an indication of the BSs eligible for network MIMO
to at least one of the BSs eligible for network MIMO. Step 206 involves
receiving data on a network MIMO zone from at least one antenna of a BS
of the BSs eligible for network MIMO. The network MIMO zone is defined by
a resource allocated for master transmission under control of the BS, or
slave transmission under control of another BS eligible for network MIMO.

[0127] What has been described is merely illustrative of the application
of the principles of the invention. Other arrangements and methods can be
implemented by those skilled in the art without departing from the spirit
and scope of the present invention.